1. Field of the Invention
The present invention relates to an image sensor, and more particularly to an image sensor adapted for causing a solid-state imaging device, in the scanning of an image, to release in succession a dark current signal generated in the solid-state imaging device and a scanning signal containing said dark current signal, subtracting said dark current signal from said scanning signal by a differential circuit to obtain an image signal corresponding to said image, and so regulating, by means of an integrating time control circuit and in response to the level of said image signal, the image signal integrating time of said solid-state imaging device that said image signal remains within a determined level range.
2. Description of the Prior Art
Solid-state imaging devices such as charge-coupled devices, recently employed in various applications, are associated, in the output signal thereof, with a noise component resulting from dark current in such devices, and are often unable to exhibit proper performance because of such noise. In an image sensor utilizing such a solid-state imaging device it is therefore necessary to detect such noise component resulting from dark current and to eliminate such noise component from the output signal, and various methods have been proposed for this purpose. For example there is already proposed a technology for so-called dark current compensation in which a part of the photoreceptor area of the solid-state imaging device is masked to obtain the dark current component in said masked area of the solid-state imaging device, and the thus obtained dark current component is retained and subtracted from the scanning signal obtained in the unmasked area of said device.
On the other hand, the solid-state imaging device requires control of the charging accumulating time or optical signal integrating time in order to expand the dynamic range in response to the luminance of the incident light, and there are already known certain technologies for this purpose. As an example there is proposed a method of comparing a certain level, for example the peak level, of the scanning signal obtained from the solid-state imaging device with a determined voltage level and reducing or extending the integrating time respectively when said signal peak level is higher or lower than said voltage level, thereby maintaining the peak level of the scanning signal within said voltage level range.
However, the aforementioned dark current compensation for the scanning output signal from the solid-state imaging device and the control for the signal integrating time in response to the level of the scanning signal, if employed in combination, may lead to an inconvenience as explained in the following. For example, in case of a sudden and rapid increase in the intensity of incident light to the photoreceptor area during the integration of optical signal by a solid-state imaging device with a relatively long integrating time controlled by the integrating time control circuit, the aforementioned masked area receives increased light leakage to accumulate a significantly increased charge therein, thus greatly raising the level of dark current signal retained in the dark current latch circuit at the signal readout from said solid-state imaging device, whereby the level of the scanning signal after the elimination of the dark current component by a dark current compensating circuit, usually composed of a differential circuit, remains almost the same as before or becomes very low. As the result the peak level of the scanning signal detected by the peak detecting circuit also remains almost unchanged or becomes very low, so that the signal integrating time of the solid-state imaging device, which should be shortened in response to the increased light intensity, remains fixed to the above-mentioned long integrating time or even changed to a longer integrating time. Such inconvenience is naturally not limited to the case of providing masked areas for dark current component detection in a part of the photoreceptor of the device, but is observable also in a case of detecting the dark current component from the charge generated in the analog shift register for charge transfer in the charge coupled device, since a rapid increase in the intensity of incident light to photoreceptor may cause the so-called blooming phenomenon, leading to the charge diffusion into said analog shift register, or may directly generate charge therein by photoexcitation, thus increasing the dark current component. Also such inconvenience is not limited to the case of identifying the integrating time in response to the peak level of the scanning signal as explained in the foregoing, but occurs also in a method relying on the average value of the scanning signal or in a method relying on the counting of "1" or "0" of the binarized scanning signal as disclosed in the U.S. Pat. No. 4,004,852. In this manner said inconvenience is inevitably inherent to the use in combination of the dark current compensation for the scanning output signal from the solid-state imaging device and the control of signal integrating time in response to the scanning signal obtained from said device.